a new series of researches opens up; we descend to the study of details and try to unravel all the complexities of natural phenomena. To enforce this point we may say that in our atmosphere every local weather phenomenon results from the interaction of the following seven forces:
1. The diurnal rotation of the earth on its axis.
2. The annual revolution of the earth in its orbit.
3. The attraction of gravitation holding the atmosphere to the earth.
4. The centrifugal force resulting from the rotation of the earth on its axis, and due to the inertia of the moving masses of air.
5. The molecular forces known as heat, light, chemism, electricity and radiant energy received by radiation from the sun with all the variations depending upon latitude, diurnal rotation and annual revolution.
6. The loss of heat by radiation from the earth and atmosphere.
7. The irregular expansions due to the irregular distribution of heat in the atmosphere which depends on the distribution of continents and oceans, and the presence of an easily condensable vapor like steam mixed with the permanent gases, nitrogen and oxygen that form the great mass of the atmosphere.
You will see from this brief and partial enumeration that we have to do with very complex combinations of phenomena, and that the results must vary with every slight variation in any one of these forces. Worse than this, we have not yet been able to observe or investigate the boundary between our atmosphere and the illimitable planetary space beyond. We know not whether gaseous particles are being added to and removed from this outer boundary. We know not whether our outer layer of atmosphere experiences any resistance from the cosmic ether as the earth rushes along in space. There are many speculations as to the origin of the earth's atmosphere; not only do these belong to geology, cosmic physics or cosmology, but they also lie at the foundation of meteorology. In the present state of our knowledge these are merely speculations; dynamic meteorology passes them by in silence and assumes that the atmosphere is now unchangeable as to its composition and mass. But who knows how soon the day will come when we shall have to recognize that a change has taken place! From this point of view I should say that in logical order the first problem for future study bears on the condition of the outer layer of our atmosphere, and in fact, my predecessors in this course of lectures, Professor R. S. Woodward, of Washington, and Professor J. H. Jeans, of Princeton, have already touched upon this question. More than that. Dr. C. C. Trowbridge, of Columbia University, has brought together many interesting facts relating to the trains left behind by meteors or shooting stars as they rush through the upper air. These meteors become visible at altitudes as high as 120 miles, showing that at that elevation there are obstacles of a
